Most of the presently used documents, such as, for example, bank notes, credit cards, identity cards, rail or airline tickets, checks and alike can be forged by means of modern copying methods at not too great an expense. Many proposals have been made having as an object to record on such documents machine-readable authenticating features, which increase the expense for anyone attempting to forge such documents, and thus reduce the probability of forgery. The probability of forgery is further reduced, if authenticating features in the form of optical microstructures are embedded in the document, which diffract light reaching the document in a characteristic manner. Such microstructures, such as, for example, holographically manufactured structures, phase diffraction gratings and kinoforms can only be manufactured at a high technical expense, and cannot be reproduced with conventional reproduction processes.
Microstructures are immediately impressed or embossed on a substrate where documents include substrates of thermoplastic material as taught, for example, in British Pat. No. 1,502,460. Greenaway, in U.S. Pat. No. 4,184,700 teaches the coating of paper documents with a thermoplastic coating or layer prior to the embossing of microstructures with optical markings. Greenaway, in U.S. Pat. No. 4,119,361, teaches the use of protective layers in the form of a laminated foil to protect a microstructure, while a lacquer layer is disclosed in Swiss Application 6382/80-9 to protect a microstructure. The protective layer serves the further purpose of hiding the microstructure from the human eye.
When subjecting such documents to authentication tests by machine, the microstructure is illuminated by a bundle of read-rays, the rays having a predetermined wavelength, or a predetermined narrow wavelength region. One or several components of the read-rays diffracted by the microstructure are sensed by means of a light sensor, and processed by an electronic evaluation device test to determine whether the intensity or the intensity ratio of these components lies within expected limit values, as taught by Greenaway in U.S. Pat. No. 4,129,382.